The invention relates to a method of, and an apparatus for, sinking bore holes, in particular exploratory and extraction bore holes.
Exploratory bore holes are intended for the purpose of examining deposits and to make it possible to take samples of the material present in the deposit. Exploratory bore holes are sunk, in particular, when the deposit is at a considerable depth and/or bodies of water lying above the deposit, such as seas or oceans, preclude the possibility of sinking exploratory shafts.
Extraction bore holes serve the purpose of loosening deposit contents from soil strata. An example for extraction bore holes which may be mentioned is the extraction of marine sediments with diamond inclusions deposited on the sea bed. Such diamond-containing deposits have usually formed upstream of estuaries in the form of reasonably sized layers spread out over a rocky base. For extracting the diamond-containing sediment material, use is usually made of apparatuses which comprise a drill head which is lowered to the sea bed with the aid of an extensible drill-pipe string extending from a floating platform. Floating platform is to be understood as any arrangement of which the height above the sea bottom or ocean floor depends on the water level at any one time. Drilling vessels are particularly suitable here.
In order to make the drill head rotate, this rotational movement being required for the drilling operation, either said drill head is arranged in a rotationally fixed manner on the drill-pipe string and the latter is made to rotate with the aid of a rotary drive (powered rotary head) arranged above the platform, or the drill string is mounted in a rotationally fixed manner on the platform and there is provided a drive which makes the drill head rotate with respect to the drill string.
The overburden which is loosened during the drilling operation is usually delivered up to the platform, through the interior of the drill string, by so-called xe2x80x9creverse circulationxe2x80x9d xe2x80x94for example by using the known air-lift methodxe2x80x94and at the platform is fed, via a pipe bend which is provided at the top end of the drill string and is in connection with the inner volume thereof, to arrangements in which diamonds and overburden are separated in a known manner.
Since the prerequisite for a satisfactory drilling result is that the drill-head end side which is equipped with at least one cutting tool always butts throughout the drilling operation, with an approximately constant advancement pressure against the end side of the bore hole, the platform comprises an arrangement which allows a vertical movement of the platform, called for example by sea swell, without this resulting in the advancement force fluctuating to any great extent or in the drill head being lifted off from the sea bed at all. Such arrangements comprise pneumatic or hydraulic/pneumatic piston/cylinder arrangements which are usually connected to a relatively large equalizing volume and are connected, for example, on the cylinder side to the drill string and, on the piston side, to an arrangement which bears the drill string, for example a drilling derrick. The equalizing volume connected to the cylinders means that the platform can execute vertical movements without the force with which the drilling tool butts against the sea bed changing to any considerable extent. If the fastening point of the piston/cylinder arrangement on the platform side is located beneath those on the drill string, then it is possible, by adjusting the pressure in the equalizing reservoirs, for the contact pressure of the drill head on the sea bed to be adjusted to a desired value from a maximum of the resultant weight.
Although such arrangements for equalizing vertical movements of the platform have frequently been used for some time now, it is disadvantageous that the drill string has to project out of the top side of the platform at least by half the maximum expected displacement of the vertical movement of the platformxe2x80x94in relation to the central position thereofxe2x80x94and it is also only at this height that it is possible to fit the pipe bend which is necessary for discharging the overburden to the separating arrangement. This is disadvantageous because, on the one hand, the center of gravity of the platform is displaced upward to a considerable extent, as a result of which the period in which the weather is favorable enough for use of the platform is reduced, and, on the other hand, the overburden has to be delivered up to the level of the pipe bend. This is associated with high energy outlay, in particular, when the platform is located at its lowest level, for example in a wave trough, since the energy outlay depends on the height of the pipe bend above the water surface.
U.S. Pat. No. 3,319,726 discloses an apparatus which is intended for sinking bore holes in the sea bed and in the case of which, in order to equalize the vertical movement of the platform, the drill string comprises two telescopically interengaging drill-string parts which can be displaced axially relative to one another. Serving for mounting the inner and outer drill-string parts is a high-outlay arrangement which comprises a plurality of seals and floating pistons for pressure equalization, is filled with a hydraulic fluid and is fully encapsulated toward the outside against the penetration of sediment. The disadvantage with this apparatus is that it involves high outlay to produce.
Furthermore, U.S. Pat. No. 3,259,198 and U.S. Pat. No. 3,669,993 disclose apparatuses in the case of which the drill string comprises a telescopic part which is intended to equalize, inter alia, the movement induced by the sea swell. In the case of these apparatuses, the drilled maternal is delivered upward via the interior of the drill string by fresh water being fed to the bore hole under pressure. For this purpose, the drill string contains two concentrically arranged tubes which form an annular gap between them. The fresh water is fed by way of the annular gap, and the drilled material is transported away by way of the inner tube. The drill strings of these apparatuses too involve higher outlay to produce.
The object of the invention is thus further to develop a method and an apparatus of the abovementioned type so as to improve on these disadvantages.
This object is achieved by an embodiment of the method and by an embodiment of the apparatus set out below.
Since, in the case of the method according to the invention, the drill string comprises at least two telescopically interengaging drill-string parts which, during a vertical movement of the platform, induced for example by the sea swell, execute a movement relative to one another, it is no longer necessary for the drill string to project beyond the platform by at least half the maximum expected vertical deflection of the platform; rather, it is possible for the top end of the drill string to be located directly above the platform surface. As a result, on the one hand, this considerably reduces the height of the center of gravity of the platform, with the result that, in relation to those platforms in the case of which the vertical movement is equalized above the platform, it is also possible to use this platform in rough seas, and, on the other hand, the now likewise lower pipe bend considerably reduces the delivery height which is necessary for lifting the overburden, with the result that, with the energy requirement remaining the same, the delivery quantity can be increased or else, with the delivery quantity remaining the same, the energy requirement can be lowered. The negative pressure which prevails in the interior of the drill string when use is made of a reverse circulation method, preferably the air-lift method, which is know per se from FR 2523205 for example, means that a certain quantity of surrounding water is always sucked in through the annular gap which is inevitably located between the telescopically interengaging drill-string parts, so that there is always water circulating around this region, as a result of which any sediment parts which may have penetrated are always washed out. It is thus possible to dispense with seals and specially protected mounting arrangements.
In the case of one possible embodiment of the method according to the invention, the drill head is arranged in a rotationally fixed manner on the drill string. In order to produce the rotation of the drill head, there is provided on the platform a drive which makes the drill string rotate.
In the case of one of the preferred embodiments of the method, however, the drill string is mounted in a rotationally fixed manner on the platform and the drill head is made to rotate relative to the drill string. By virtue of this configuration, on the one hand, the drive which increases the weight of the platform and possibly displaces its center of gravity upward is rendered superfluous and, on the other hand, it is possible for the drill string to serve for routing electric, hydraulic or similar lines without high-outlay rotary lead-throughs or couplings being required for this purpose. It is then also easily possible for one end of a cable which is routed to the platform to be fitted on the bottom drill-string part, as a result of which it is possible, without high energy outlay, for the bottom drill-string part, together with the drill head, to be lifted, for example, from the bottom of the bore hole and displaced over the sea bed to the location at which a further bore hole is to be sunk.
The apparatus-related aspect of the invention is one preferred embodiment.
In the case of on of the preferred embodiments, the drill string comprises an outer and an inner drill-string part, it being the case thatxe2x80x94in order that the vertical movement of the platform can be equalizedxe2x80x94the inner drill-string part is pushed into the outer drill-string part in the normal position of the platform, at least over a length which corresponds to the maximum upward vertical movements of the platform which are to be expected as a result of sea swell or the like, and can be displaced further into the outer drill-string part, at least by an amount which corresponds to the maximum expected downward deflection of the platform out of its normal position.
Since the inner and outer drill-string parts have to be connected to one another in a rotationally fixed manner, in order to be able to transmit the drive torque or the reaction torque, a configuration of the drill string including rails and sliding elements is recommended. This ensures that, despite the fact that they are fixed rotationally, a low-friction movement of the two drill-string parts relative to one another is possible.
A particularly preferred technical configuration of the mounting which works on this principle forms the subject matter of other preferred embodiments as set out below.
It is possible, on the one hand, for the outer drill-string part to be arranged on the platform by way of its top end and for the drill head to be arranged at the bottom end of the inner drill-string part. In the case of one of the preferred embodiments, however, the arrangement is the other way round, since the top, preferably longer top drill-string part, which during the drilling operation, in a vertical direction, is connected rigidly to the platform, comprises components which involve less outlay to produce.
In the case of a first possible embodiment of the apparatus according to the invention, there is provided on the platform an arrangement with which the drill string can be provided to rotate about its longitudinal axis. In this case, the drill head is arranged in a rotationally fixed manner on the drill string.
In the case of one of the preferred embodiments. however, the drill string is mounted in a rotationally fixed manner on the platform and there is provided a rotary drive, with the aid of which the drill head can be made to rotate with respect to the drill string.
In the case of the above mentioned preferred embodiment the rotationally fixed mounting of the drill string on the platform preferably takes place by means of a rotationally fixed, cardanic retaining means (gimbal).
The drill head is preferably driven either electrically or hydraulically, it being the case that the drive arrangements are preferably integrated in the drill head or are arranged directly above the same.
The pressure line which is required for using the air-lift method may, according to an alternative embodiment, be secured on the outer circumference of the drill string and run parallel to the longitudinal center axis thereof.
In the case of an advantageous embodiment, however, the pressure line is designed as a pressure hose which can be unrolled from a winding drum. This measure has the advantage that the pressure hose can easily be connected both to the top drill-string part and to the bottom drill-string part.
An advantageous development of the apparatus according to the invention forms the subject matter of an alternative embodiment. In this case, at least one buoyancy element is arranged on the drill-string part which bears the drill head. In the case of the apparatus according to the invention, selection of the buoyancy volume of the buoyancy element makes it possible to adjust the desired contact pressure of the drill head on the sea bed.
In order to avoid the situation where the bottom drill-string part is subjected to compressive loading by too high a buoyancy volume of the buoyancy element, it is advantageous, according to an alternative embodiment, for the at least one buoyancy element to be arranged in the vicinity of the top end of the bottom drill-string part.
Straightforward adjustment of the contact pressure, this adjustment also being possible once the drill head has been lowered onto the sea bed, is provided if the buoyancy elements are designed as floodable tanks which can be filled with compressed air as desired, via corresponding compressed-air lines, possibly by a compressor located on the platform.
If, according to an alternative embodiment, a buoyancy element is also provided on the drill-string part which is mounted on the platform, it is possible to compensate for some of the weight acting on the platform, with the result that heavier, and thus longer, drill-string parts can be secured and the apparatus is also suitable for sinking exploratory bore holes at considerable depth.